构造地质学双语joints.docx

构造地质学双语joints.docx

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构造地质学双语joints

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[OPPTR-OPPT28-OPPTL98-OPPNN08]

构造地质学双语joints

Chapter4Joint

Keywords

Beddingjoints

Strikejoints

Dipjoints

Obliquejoints

Fractures

Diagonaljoint

层节理

走向节理

倾向节理

斜向节理

破裂

斜节理

Jointsets

Jointsystem

Systematicjoints:

Non-systematicjoints

Plumosestructure

节理组

节理系

系统节理

非系统节理

羽饰构造

Shearjoint

Tensionjoint

en-echelon

Longitudinaljoints

Transversejoints

剪节理

张节理

雁列（脉，节理）

纵节理

横节理

Hackle

plumeaxis

tensiongashes

Regionaljoint

Stylolites

羽脉

羽轴

张裂脉

区域性节理

缝合线

Fracturesaresurfacesalongwhichrocksormineralshavebroken.Joints,beoneofthemostcommonlyfoundpervasivemesoscopicstructure,arethefractureswithoutapparentdisplacementinrocks.Thesediscontinuitieswillbeencounteredinmanyengineeringtasksinvolvingnationalandlocalneeds.Cracksinrockscarrygroundwaterfarmoreefficientlythanthebulkrockwithitsrelativelylowpermeability.Incontrasttherecoveryofgeothermalenergyrequirescracksinhotrocksothatheatfluidspumpedalongthecrackscanactasthemediumwithwhichheatedwateristransportedtothesurface.Therecoveryofpetroleumisbestaccomplishedwhenwellscutafracturednetworkcontaininglargeamountsofpetroleum.Theproperty,orientationanddistributionofjointsarecloselyrelatedtothefolds,faultsandregionalstructures.So,thestudiesofjointsareveryhelpfultoanalyzethegeologicalstructure.

1.Classificationofjoints

Theclassificationofjointsisbasedontwomainfactors:

（1）thegeometricalrelationshipwithotherrelatedstructures.

（2）Mechanismsofthejoint’sforming.

Jointsarerelativelybelongingtosmall-scalestructures.Theorientationsofjointsoftenhavegeometricalrelationshipwithotherstructures.

（1）A:

Therelationshipbetweenjointsstrikeandsenseofassociatedstrata.

Strikejoints.Strikeofthejointisparalleltothestrikedirectionofbed.

Dipjoints.Thestrikeofthejointisperpendiculartothestrikeofbed.

Obliquejoints.Thestrikeofthejointisobliquetothestrikeofbed.

Beddingjoints.Jointplaneisparalleltothebeddingplanes.

B:

Therelationshipbetweenjointandfoldaxis

Longitudinaljoints:

Strikeofthejointisparalleltothefoldaxes.

Transversejoints:

Strikeofthejointisperpendiculartofoldaxes.

Diagonaljoint:

Strikeofthejointisaobliquetofoldaxes.

（2）Accordingtothemechanismofjoints

Shearjoint.Incompression,jointsdevelopintheconjugatesheardirections（theorientationofsymmetricfractureplanes）makingtheloweranglewiththemajorprincipalstressdirection.Shearjointsareoftengrooved,striated,polishedorslickensidedbyevensmallamountsofsheardisplacement.

Conjugateshearfracturesmaybeusedtomapmaximumprincipalstressinthevicinityofafaultzone.

Shearjoint

Tensilejoint

Intension,jointsdevelopbystretchingnormaltothetensilestressdirectionwhichisusuallytheminorprincipalstress.Tensionjointsarerough（unlesssubsequentlyweathered）.Incoarsegrainedrockssuchsurfacesmaybeveryrough.Tensionaljointincludecolumnarjointsinbasalts,dykesandsills,tensiongashes,calciteandquartzfilledveins.Extensionjointsarenotonelongdiscontinuitybutratherseveraljointsthatformend-to-endinajointzone.

（3）Jointsetsandjointsystems

Jointsets:

manyadjacentjointswithsimilargeometryandorientation.Systematicjoints:

roughlyplanar,sub-parallelorientations,regularspacing.Non-systematicjoints:

curvedandirregularingeometry,usuallyterminateagainstsystematicjoints.

Jointsystem:

madeupoftwoormoresetsofjoints,maybepersistentoverlargeregions.Thegeometryofjointsystemsaredescribedwiththefollowingmeasurementsandobservations:

orientation,scaleandshape,spacingandaperture,intersectionsandterminations,spatialpatterns.

3.Surfacemorphology

Somejointsurfacesdisplaybeautifulsurfaceornamentation,thatisplumestructure.Plumosestructureconsistsofanumberofmorphologicalcomponents:

plumeaxis,hackles,andfringezone.Curvedhacklemarksradiateoutwardfromthepoint（origin）wherethejointoriginated.Thesestructuresindicatethepropagationdirectionofthejoint.Thefringeofajointmaybemarkedbycloselyspaceden-echelonjointsobliquetothemainjointface.

3.Enechelonveins

Veinsarefracturefilledwithaprecipitate,commonlyquartzorcalcite,rarelymud.Enechelonveiniskindofveinwhichcanbefoundinmanyrocks,especiallyincarbonaterocks.Enechelonareparallelorsubparallel,closely-spaced,overlappingorstep-likeminorstructuralfeaturesinrock,suchasfaultsandtensionfractures,thatareobliquetotheoverallstructuraltrend

Sigmoidaltensiongashes,generallyS-orZ-shaped,formalongzonesofductileshear.Thistypeofextensionalfractureisusuallymineral-filledandformenechelonarraysalongtheshearzone.EnEchelontensionfractures（gashes）mayindicatethedirectionofdisplacementbytwofeatures:

1）Sigmoidalprofilesoffractures,and2）offsetdirectionofvein-filledfractures"

Thesigmoidalprofilesoffracturesarecross-sectionalviewsthataidindeterminingthedirectionofrotation.TheprofilesresembleanS-shapedfeaturewiththetipspointinginthedirectionofrotation,eitherclockwiseorcounter-clockwise.

Thegashfracturesformasextensionalfracturesthatareperpendiculartotheminimumcompressivestress,sigma3（seediagram）.Thefracturesmayberotatedbyductiledeformationeitherduringorafterformation.Asgashfracturesdevelopatdifferenttimesofductileshearingtheyshowdifferentamountsofrotation.Sincesigma3isnormaltotheunrotatedportionofthegashfracture,eitherthetipsofthesigmoidalfractureorthenewlyformedfracturestellsthestressorientation.Theyoungestfracturesmaynotshowanyorverylittlerotation.（seediagram）.

4.Stylolites

Stylolitesareirregularsurfacesthatcommonlyappearasdark,jaggedlinesonexposedsurfacesofcarbonaterock.Theiroriginisusuallyattributedtosolutionthatoccursafterthehostrockwasformed.Thedarklayersareinsolubleresidues.Stylolitesoftencrossthewholerock,cuttinggrains,matrixandcementstylolitesbelongtopressuresolutionstructuresformedatstrainlocalisationsiteswithinrocks,andcanbeusedtoindicatethestressdirection.

4.RelationshipsBetweenJointsandOtherStructures

Columnarjoints:

polygonalfracturesformedduringcoolingoflavaflowsandshallowintrusions.Duetotensilestressessetupasigneousrockcontractsduringcooling.

Sheet（orexfoliation）joints:

curvedjointsthataresub-paralleltothetopography.Earthsurfacesiserodedandrelievesverticalstressbutlateralstressisnotreducedproportionally.Thereforeverticalstressbecomesminimumprincipalstressandjointsformperpendiculartolandsurface.Shrinkageofcoolingplutons（largehomogenousigneousmasses）mayalsoproducesheetingjointsorexfoliation.

Jointsassociatedwithfolds:

Severalsetsofjointsmaydevelopinresponsetofolding.Conjugateshearjointsobliquetothefoldaxisaredevelpoedbycompression.Tensionjointscandevelopduetobending,particularlyinthevicinityofthefoldhinge.Jointsparalleltothestrikeofthefoldaxisarecalledstrikejoints;thoseparalleltothelimbdipareknownasdipjoints.Suchjointsaregenerallytensilefractures.

Jointsassociatedwithfaults:

highstresslevelsinthehangingwallandfootwallofafaultcanresultintheformationofconjugateshearjoints,withonesetparalleltothefaultplaneandtheothersetatanobliqueangleofabout65?

.

Jointsinplutonicrocks:

severaljointsetsarecommonlyobservedinplutons.Suchjointsarethoughttoberelatedtostressessetupduringcoolingoftherockmasswithinaregionalstressfield.

Regionaljointsets:

remarkablyconsistentsetsofjointsareobservedattheregionalscale,particularlyinweaklydeformedsedimentary"platformsequences".Suchjointsareusuallytectonicjoints:

thatisextensionaljointsformedduetohorizontaltectoniccompressionunderconditionsofelevatedporefluidpressure.

withJointsandVeinsintheField

Studywayofjointsvarieswithdifferentpurposes,butingeneral,allofthestudiesonjointsarebasedonthemeasurement,observation,andstatistics.

（1）Localities.Theobservationlocalitiesshouldhavethefollowingfeatures:

Wellexposedoutcrops

Clearlyobservablestructure

Beddingswithtableorientation

Clearrelationshipofjointsetsandjointsystems.

（2）Contentsofobservation

Geologicalbackgroundobservation.Includingtheorientationandrocktypesofbeddings,thefeaturesoffaultandfolding.

Classificationofjoints,jointsetsandjointsystems.Describingtheintersectionandterminationofdifferentjointsets.

Timing:

Youngerjointscommonlyterminateintoolder,existingjoints.Becausefracturestopsatfreesurfaceofthepreexistingjointandcannotpropagateacrossit.Also,jointstendtobecomeparallelormorecommonlyperpendiculartothefreesurfacebecausethefreesurfacecannotsupportshearstress.

Measurementofjointdensity.Thejointdensitycanbemeasuredbyjointnumbersinunitlengthortotalunitlengthsinunitarea.

Observationofjointsurface.Specialattentionshouldbepaidtostriation,plumestructureandothersmallscalestructures.

Whetherthejointhasbeenfilledwithminerals.Thedistributionandscaleofveins.

（3）Records

Usingspeciallydesignedtablestorecordstheresultofmeasurementandobservation.